Ink jet printers involve the use of liquid-based inks that can be jetted onto a receptor, often a sheet of paper or film, to produce an image. The use of four basic ink colors (cyan, magenta, yellow, and black) in various combinations and concentrations allows virtually any color to be produced as part of a printed image. Ink jet technology is well-suited for high resolution graphic images, particularly those produced using digital printing systems. Such systems typically employ computer technology to create, modify, and store images, text, graphics, and the like.
Inks suitable for ink jet printing applications must meet requirements that can be quite different from inks used in other conventional printing technologies. The viscosity of ink jet inks must be relatively low, e.g., typically less than about 20 centipoise (cp). At the same time, ink jet inks preferably have a controlled surface tension to control droplet formation. The inks or their ingredients should not clog thermal or piezo print heads (e.g., at the nozzle) upon standing or during the printing process. And, the ink should dry quickly to a flexible and abrasion resistant image. Fast drying is important because it is a key factor in determining printer productivity.
Many inks used in the past with ink jet printers comprise dyes contained within carrier liquids. Although such inks may offer satisfactory performance in certain applications, dye-based ink systems can tend to produce images that lack light stability and durability properties needed for outdoor and similarly demanding applications.
Inks prepared with pigments as colorants instead of dyes have been considered. In pigmented inks, different colored pigments can be dispersed into pigmented dispersions for use in ink jet printers. Ink jet printheads employ extremely fine printhead nozzles. The size of the pigment particle must therefore be small, and the particle size distribution sufficiently narrow to avoid plugging of the nozzles by the pigment. Also, the pigment particles within the ink should be in a stable state of dispersion; the pigment particles should not agglomerate or flocculate, causing the pigment particles to settle out of the ink dispersion. Additionally, in the case of thermal ink jet systems, pigmented ink dispersions suffer from the tendency of materials to settle onto, and coat, the heating elements inside printer head nozzles. This causes a reduced thermal efficiency of the print head which results in the formation of smaller ink droplets, lower drop velocity, and lower image quality. This effect is commonly referred to as "kogation".
While these properties are often desirable or even necessary, their achievement can be difficult in pigmented inks. Pigment dispersions are inherently thermodynamically unstable. Suspended pigment particles can tend to agglomerate due to attractive interactions (e.g., van der Waals forces) between them.
Dispersants are typically employed in pigmented ink jet inks to stabilize the dispersion to prevent agglomeration or flocculation of the pigment particles and consequent settling of the pigment particles out of the dispersion. It is theorized that dispersing compounds behave by adsorbing onto a pigment surface to create a protective layer around each pigment particle to counteract attractive forces between the particles, thereby preventing agglomeration and flocculation.
Typically, four or more colors are used for ink jet printing. The different colored inks, and the components thereof, are preferably compatible with each other when in contact prior to drying. Yet, different colors and types of pigments tend to have different surface properties, and it can therefore be difficult to form inks of different colors, in stable dispersions, using the same dispersant. A set of multi-colored inks may require several different dispersants. A problem exists when the different dispersants are optimal for different colored pigments, leading to a high risk of incompatibility. For instance, different colored inks are considered incompatible if, upon mixing, a component of an ink (e.g., a dispersant) were to cause flocculation of pigment particles of another ink, causing portions of the colored image to be inaccurate or otherwise unacceptable.
A need exists for dispersants useful in ink jet inks that exhibit one or more of the above-described properties, including: the ability to disperse a pigment into a stable dispersion that will not suffer from agglomeration, flocculation, or kogation; preferably, the ability to be used with a number of different colored pigments comprising a broad spectrum of colors; that do not cause significant deposition of substances on heater elements during jetting; that provide an ink having fast drying properties; and, that provide an ink of proper character (e.g., viscosity, surface tension, etc.) for use in ink jet printing. A need simultaneously exists for ink jet printer inks that are free or substantially free of organic solvents.